Ohh I see now! In 3D, I've seen the use of grayscale SDF masks to control highlights in a similar way, particularly on the modern toon-shaded anime styles. In 2D, I've started to use black and white textures for alpha masks for particles and other mass instanced sprites, and randomize their color in the shader.
It's so cool how normal mapping makes 2D art pop as if it was low-relief sculpture. Something I've been exploring is using software like Material Maker to generate diffuse, normal, and specular maps for use on things like walls, similar to this video. It's actually quite viable, but I'm not fluent in the node system yet.
re: film grain, I think it's OK if you like the way it looks stylistically. Some games offer an option to turn it on or off, for example Okami and Neva. I only notice because I've been studying dithering and how to use it to fix color banding, which I've had some problems with in gradients in Godot. Maybe Unity handles it better automatically?
Thanks for sharing these resources and your knowledge!